I think you meant David Allen, rather than Cal Newport.
Cal Newport seems to think GTD is flawed because it doesn't help you prioritize meaningful work: http://calnewport.com/blog/2012/12/21/getting-unremarkable-t...
Cal Newport seems to think GTD is flawed because it doesn't help you prioritize meaningful work: http://calnewport.com/blog/2012/12/21/getting-unremarkable-t...